1. Field of the Invention
The present invention is directed to lower limb prosthesis. More particularly, the present invention is directed to the adjustment of lower limb prosthesis to suit the ambulatory characteristics of the amputee to which it is to be fitted.
2. Background of the Related Art
Lower limb prosthetic devices are presently comprised of two basic classifications. One classification, termed exoskeletal prosthetic devices, relates to devices where the vast majority of the vertical load of the amputee is supported by the shell of the prosthetic appliance. This shell has basically the same exterior shape of the lost limb, and is usually manufactured from a combination of wood and rigid thermoset plastics reinforced with some type of long fiber, to which a separate plastic foot may be attached. This construction renders the prosthesis hard to the touch, but it may be coated with a thin layer of flexible material for cosmetic reasons.
The second type of lower limb prosthetic appliance is termed endoskeletal. This classification relates to a device which carries the vast majority of the vertical load through an inner supporting member. This inner member, or framework, has a shape totally unrelated to that of the missing limb. This inner structure is subsequently covered with a light weight flexible foam, which is then given the proper shape. This structure may or may not include an attached prosthetic foot. If a prosthetic foot is not included in the basic design, a separate prosthetic foot is attached.
An AK (above knee) prosthetic appliance most always includes a knee joint, which is the axis about which the shin portion of the prosthesis rotates. A BK (below knee) prosthesis would usually not contain this knee joint, and would be used in cases in which the length of the residual limb is sufficient to support a prosthetic appliance.
A below knee (BK) endoskeletal prosthesis can further be divided into two smaller classifications. As of this date, they are not specifically named, but can be described as follows. The more popular prosthetic device, which is accepted as the worldwide standard, is comprised of separate foot and shin members, which may be fabricated from components of different manufacturers due to modular design features that exist in much of the componentry available today. This appliance consists of a rigid rod or tube made of metal or thermoset composite material (with the reinforcing fiber always being glass or carbon) attached to a custom fabricated cup or socket which contains the residual limb. At the other or lower end of this rigid structural tube is the prosthetic foot.
Walking is a complex movement which involves not only the pivoting of the lower limb around the knee joint, but also a rotation of the lower limb about its length. It is also necessary to obtain an optimal position of the foot relation to the prosthetic socket using the "trial and error" approach. For this reason, lower limbs are not perfectly straight but are slightly bent and twisted by amounts unique to a given individual. It is therefore common practice to provide lower limb prostheses with angular adjustments tailored to the comfort and gait of the individual wearer. For this purpose, before receiving a permanent prosthesis the amputee is given a "temporary leg." This is a somewhat cumbersome device containing many adjustable features to help determine the proper alignment for the amputee. After a sufficient amount of time walking on the temporary leg, the prosthetist is able to determine the proper relationship between the prosthetic foot and the socket of the residual limb. This temporary leg is then placed in an apparatus called a bench mounted alignment fixture, a device common to the industry. The socket, which can be reused, is positioned and held firmly by this device and the position of the foot relative to this socket is then noted and recorded The temporary leg, with the exception of the socket, is removed from the bench mounted alignment fixture, making the socket ready for the installation of a permanent prosthesis.
The conventional permanent prostheses all use angularly adjustable adaptors at the ends of a rigid rod. An example may be found in British patent specification 978,586 which uses a pair of lockable ball joints at the ends of a tube and connecting the tube to both the prosthetic foot and the socket. Although such adaptors (e.g., ball joints) give the prosthesis the advantage of adjustability, they have two pronounced disadvantages. They add weight and allow the possibility of failure due to the mechanical fasteners they contain becoming loosened during normal usage.
Additionally, in order to allow limited rotation about an axis parallel to the length of the tube, a separate device for rotation should be added. However, such a rotator, although it is highly desirable since it reduces the frictional forces generated between the residual limb and the socket, is seldom used because of the resulting additional weight. Additional weight increases the swinging mass of the leg and requires additional force to maintain the prosthesis firmly attached to the residual limb. Such additional force almost always irritates scar tissue in the area of the amputation and/or further restricts the flow of blood to the most critical areas.
An additional shortcoming of conventional lower leg prostheses is the undesirable transmission of vibrations from the foot, through the tube or rod of the prosthesis, and to the residual limb.
The second or hybrid classification of existing BK prosthesis is that shown in U.S. Pat. No. 4,865,612, which contains a shank and foot constructed of one continuous member. In this case, the supporting inner member is not a tube but is of a solid, flat rectangular cross section, the wider portion running side to side. This single member design continues with a smooth radius in the area of the ankle and out to the toes. The material from which it is manufactured is basically a combination of carbon and fiberglass fibers, the majority of which run in the direction from prosthesis socket to prosthetic toe, contained in a thermoset epoxy matrix. This renders the prosthesis relatively rigid, but does allow bending under load. This bending under load takes full advantage of the composite materials by storing the energy input to the system due to the bending forces, and returning it at a later point in the gait of the amputee. Such energy storage and release is advantageous to more active amputees such as joggers and other sports enthusiasts, but is of little assistance and possibly even detrimental to the majority of amputees. The ability of this construction to allow sufficient rotation about the vertical axis is also extremely limited. This is primarily due to the cross section area being rectangular in shape, the dimensions of this cross section, the orientation of fiber directions, and the rigid thermoset matrix that bonds the fibers together. Alignment changes are also extremely difficult and expensive. Moreover, an added attribute is the composite's high strength to weight ratio, resulting in a relatively lightweight prosthesis. The importance to the amputee of this weight reduction over conventional prosthetic appliance construction cannot be underestimated.